The present invention relates generally to solid state lasers and more particularly to doped or undoped semiconductor laser arrays operating at either room or reduced temperatures for obtaining higher power output and higher power density than heretofore obtained.
It has become well known that by the application of an electrical current to certain semiconductor diodes doped so as to contain a pn-junction, electromagnetic radiation or more particularly laser radiation is produced. For example, a simple gallium arsenide laser diode consists of a two-layer sandwich of semiconductor material, one layer being p-doped gallium arsenide and the second layer being n-doped gallium arsenide. The layers are positioned adjacent one another to provide a pn-junction from which, upon energization by an electric current passing therethrough, laser radiation is emitted. This may be accomplished by changing the deposit during deposition thereof to form a continuous gradient in the transition from the p-region to the n-region.
Typical laser diodes range from about 5 to 50 mils square, and about 5 mils thick. The laser light generated in the gradient between the layers emanates from a region which is between about 0.02 to about 0.08 mils thick. Various structures such as reflecting means may be included about the laser diode to allow the laser beam to emanate from only one edge thereof. Thus, the apparent laser source may be very elongated, about 5 to 50 mils long by about 0.02 to 0.08 mils high. Due to these dimensions and the non-homogeneities along the length of the pn-junction, the laser radiation is thereby emitted at a conical angle of approximately 20 degrees along two orthogonal axes.
Such laser producing diodes are being found useful in an increasing number of applications, for example, in cutting devices and range measuring systems. However, in many applications, a single laser diode is not sufficient to provide the required power intensity and, therefore, a laser source of much greater intensity is necessary. Generally, increased power can be achieved by using an array of multiple laser diodes which can be stacked and/or placed side by side. However, with such arrangements, two principal problems have been encountered. The emitted beams tend to diverge greatly and lose their intensity, while large amounts of heat are generated in the laser-active pn-junctions as well as in the neighboring regions of the semiconductor body due to the close proximity of diodes. The heat generated thereby may cause severe damage to the laser elements, especially when densely packed.
In order to minimize the loss in intensity of the laser beam due to beam divergence, systems of lenses, generally spherical, have been designed for focusing the beams of laser radiation. Although spherical lenses have been found adequate for some applications, it has been suggested that plano-cylindrical lens systems would enable the use of densely packed laser arrays for producing laser radiation of relatively high intensity. Among known lens system employing plano-cylindrical lenses, such as disclosed in U.S. Pat. No. 3,396,344 issued to Broom, an array of laser diodes is mounted to a suitable substrate in symmetrically arranged rows and columns, behind two groups of crossed plano-cylindrical lenses of relatively high collection angle (f/3 or greater), with the number of lenses in the inner group being equal to the number of vertical columns of laser diodes and the number of lenses in the outer group being equal to the number of horizontal rows of laser diodes. Although such systems have provided higher intensity laser radiation, they are generally incapable of generating well-defined or high quality laser beams because of the difficulties encountered in attempting to maintain precise alignment between the laser elements and their corresponding lenses at their operating temperatures, as well as the presence of one dimensional aberration--cylindrical aberration--analogous to spherical aberration experienced in spherical optics. In addition, the mounting of individual components of the size necessary to produce the requisite power density with individual screws, bolts, cams and pillow blocks becomes difficult if not impossible at packing densities approaching 500 or more laser diodes per square inch. Moreover, the use of a number of outer lenses of relatively long focal length and high collection angle corresponding to each individual laser limits both the packing density of the laser generating diodes and the mechanical packaging of the lens system. Furthermore, the use of fine adjustment means for aligning the system elements not only increases fabrication costs but also fails to adequately solve the alignment problem for higher intensity applications.
In order to remove the undesirable heat generated by the laser diodes, the laser generating elements are commonly mounted on a simple heat sink, or, as shown by U.S. Pat. No. 3,760,175 issued to Gibson et al., they may be mounted to the wall of a liquid coolant reservoir. However, as higher packing densities are achieved, removal of heat by such means becomes a severe limitation on performance as it cannot adequately accommodate the substantial amount of heat generated.
It is therefore an object of the present invention to provide a new and improved semiconductor laser array system.
Another object of the present invention is to provide a new and improved laser array system which is essentially permanently adapted for accurate optical alignment of the laser module elements at the pre-selected operating temperature of the system.
It is also an object of the present invention to provide an improved laser array system including acylindrical lenses which are essentially aberration-free and easily fabricated.
It is a further object of the present invention to provide an improved semiconductor laser array system capable of increased packaging density and both higher power output as well as higher average power output than heretofore achieved.
It is a further object of the present invention to provide an improved semiconductor laser array system which has a greater heat removal capacity than systems heretofore know to allow higher packing densities and operation at higher intensities.
It is another object of the invention to provide an improved semiconductor laser array capable of achieving substantially high peak and/or average power output in a narrower beam than heretofore available in a system having fewer parts and costing relatively less to fabricate.
It is still a further object of the invention to provide an improved semiconductor laser array having a heat removal system which allows continuous operation at any desired temperature irrespective of the output level of operation.
It is yet another object of the present invention to provide an improved semiconductor laser array capable of achieving an output power density per unit angle per unit source which is constrained only by the diffraction limit of the optics.
Objects and advantages of the invention are set forth in part herein and in part will be appreciated herefrom, or may be learned by practice of the invention, the same being attained and realized by means of the instrumentalities and combinations pointed out in the appended claims.